.

I doubt you will find someone here who experienced this exact situation previously. If I was in such situation, I would rather just go with a safe 20% bet, than hope for 23% but only receive 3%.

I don't know whether this would work or not but if the 3% expires in an hour, it seems you could try to apply the 20% voucher after the 3% discount and if it's rejected at checkout then don't complete the sale. Then if you wait an hour, assuming the 3% discount is expired and no longer applicable, it seems the 20% voucher should apply in this case as a backup plan.

Just a thought, you're assuming all the risk of course if you want to try to maximize the discounts. Best of luck.